1. Field of the Invention
This invention relates to laser shock peening and, more particularly, to apparatus and methods for minimizing or eliminating electromagnetic feedback during laser shock peening laser.
2. Description of Related Art
Laser shock peening (LSP) or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of an article. Laser shock peening typically uses one or more radiation pulses from high and low power pulsed lasers to produce an intense shock wave at the surface of an article similar to methods disclosed in U.S. Pat. No. 3,850,698 entitled “Altering Material Properties”; U.S. Pat. No. 4,401,477 entitled “Laser Shock Processing”; and U.S. Pat. No. 5,131,957 entitled “Material Properties”. Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force at the impingement point of the laser beam by an instantaneous ablation or vaporization of a thin layer of that surface or of a coating (such as tape or paint) on that surface which forms a plasma.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. No. 5,756,965 entitled “On The Fly Laser Shock Peening”; U.S. Pat. No. 5,591,009 entitled “Laser shock peened gas turbine engine fan blade edges”; U.S. Pat. No. 5,531,570 entitled “Distortion control for laser shock peened gas turbine engine compressor blade edges”; U.S. Pat. No. 5,492,447 entitled “Laser shock peened rotor components for turbomachinery”; U.S. Pat. No. 5,674,329 entitled “Adhesive tape covered laser shock peening”; and U.S. Pat. No. 5,674,328 entitled “Dry tape covered laser shock peening”, all of which are assigned to the present Assignee.
Laser peening has been utilized to create a compressively stressed protective layer at the outer surface of an article which is known to considerably increase the resistance of the article to fatigue failure as disclosed in U.S. Pat. No. 4,937,421 entitled “Laser Peening System and Method”. These methods typically employ a curtain of water flowed over the article or some other method to provide a plasma confining medium. This medium enables the plasma to rapidly achieve shockwave pressures that produce the plastic deformation and associated residual stress patterns that constitute the LSP effect. The curtain of water provides a confining medium, to confine and redirect the process generated shockwaves into the bulk of the material of a component being LSP'D, to create the beneficial compressive residual stresses.
High energy laser beams, from about 20 to about 50 joules, or low energy laser beams, from about 3 to about 10 joules, have been used and other levels are contemplated. See, for example, U.S. Pat. No. 5,674,329 (Mannava et al.), issued Oct. 7, 1997 (LSP process using high energy lasers) and U.S. Pat. No. 5,932,120 (Mannava et al.), issued Aug. 3, 1999 (LSP process using low energy lasers). Low energy laser beams can be produced using different laser materials such as neodymium doped yttrium aluminum garnet (Nd YAG), Nd:YLF, and others.
A combination of the energy of the laser and the size of the laser beam provides an energy density or fluence that is usually about 200 J/cm2. Laser shock peened spots are typically formed in overlapping rows of overlapping spots. Typically, overlaps of about 30% of diameters between both spots in a row and between spots in adjacent rows are used. The laser shock peened spots and laser beams are typically circular in shape but may have other shapes such as oval or elliptical (see U.S. Pat. No. 6,541,733, entitled “Laser Shock Peening Integrally Bladed Rotor Blade Edges” by Mannava, et al., issued Apr. 1, 2003.
The pressure pulse from the rapidly expanding plasma imparts a traveling shock wave into the component. This compressive shock wave caused by the laser pulse results in deep plastic compressive strains in the component. The blast wave energy travels hemispherically, partially into the article being laser shock peened behind the and partially into the confining medium such as the curtain of water. The confining medium reflects some or all of the outward traveling energy back into the article to enhance the energy coupling for LSP. Since the LSP process uses water and often the article is made of metal, the potential for electromagnetic radiation reflections from the incoming laser pulse is highly probable.
These reflections can be detrimental to both the LSP process and the optical components within the laser for several reasons. If the reflections pass through an optical amplifier such as those used in a master oscillator power amplifier (MOPA) laser system which are commonly used for LSP processing, their energy can be amplified throughout the entire laser system thus causing random uncontrolled laser pulses to be generated. These random laser pulses can physically damage the ablative and/or the article. These random laser pulses have the ability to physically damage components within the laser system, thus rendering it inoperable. Finally, these random laser pulses can reduce the energy gain of any optical amplifiers, and therefore reduce the process capability of LSP. LSP laser systems using ND:YAG and any lasing mediums with high small signal gains are extremely susceptible to adverse effects from target reflections. This problem probably applies to all lasing mediums used in the LSP process.
Thus, it is highly desirable to have a laser shock peening apparatus that includes a technique or apparatus for preventing light reflections from reentering a laser system used for laser shock peening (LSP). It is highly desirable to provide isolation of electromagnetic radiation feedback from the target during laser shock peening and prevent light reflections from reentering a laser system used for laser shock peening (LSP).